Wafers are used as the base of integrated circuit chips. As wafers increase in size, the thickness of the wafers have been increased for mechanical stability of the wafer during device processing. For example, when producing a 200 mm wafer, the wafer thickness is about 725 microns and when producing a 300 mm wafer, the wafer thickness is about 775 microns. The increase in wafer size results in an increase in the amount of a silicon bulk material which is consumed for each usable wafer area. With wafers sizes approaching 450 mm, effective utilization of a single crystalline bulk material to produce the wafer is of critical importance, especially in view of the increasing expense of raw materials as well as increased competition for polysilicon from the photovoltaic industry.
In order to reduce the thickness of a wafer, standard device processing is usually finished by mechanically grinding of the back side of a wafer. This enables the thin layer to be as close as possible to the heat sink or other heat dissipation component of the wafer. The grinding step may be performed before separation/cutting the wafer into die and finalizing of individual devices. Grinding results in the loss of a significant amount (more than 50%) of silicon (i.e., silicon which is not included in the final integrated circuit chip).
In order to maintain the wafer on a supporting surface or substrate, it is necessary to have a bond which is strong enough to survive device fabrication but weak enough to be separated from the supporting surface without causing damage to the wafer or supporting substrate. Existing techniques for forming a weaker bond require that an SiO2 layer of the wafer may be chemical or dry plasma etched. Such etching results in roughness which would make the bond weaker than a perfectly smooth surface. If the surface, however, is too rough, the wafer may delaminate during device fabrication. For example, during a wet treatment step, liquid may flow at the level of the interface between the wafer and the supporting substrate and cause uncontrolled detachment/debonding during further process steps.
It is desirable to have a wafer which requires a minimal amount of material and which may be attached to and non-destructively removed from a supporting substrate that provides the required mechanical properties during device fabrication. In this way, the substrate may be conserved for reuse with other wafers to form further devices.